Death Guard psykers have access to three psychic powers that cause direct damage:

Smite (warp charge 5) targets one enemy unit within 18", dealing D3 mortal wounds, or D6 if the test result is 11+.

Plague Wind (warp charge 5) targets one enemy unit within 18", dealing 1 mortal wound for every 6 rolled on nD6 (where n is the number of models in the target unit).

Curse of the Leper (warp charge 7) targets the closest enemy unit within 14", dealing 1 mortal wound for each roll on 7D6 that exceeds the target unit's Toughness.

The question we ask is, "Which is best?" And naturally the answer is, "It depends." Here are the details...

All three deal mortal wounds to one enemy unit. The key metric therefore is the expected number of wounds inflicted by each power.

Smite has a 27/36 chance of D3 (mean 2) wounds, and a 3/36 chance of D6 (mean 3.5) wounds. That gives an expected number of 1.791(6) mortal wounds: our base value for comparison.

Plague Wind has a 30/36 chance of manifesting an average of n/6 wounds. So Plague Wind is better than Smite only if the target unit contains 13 or more models.

Curse of the Leper has a 21/36 chance of manifesting an average of 7x(6-T)/6 wounds (0 for Toughness 6+). So Curse of the Leper is better than Smite only if the target unit has a Toughness of 3 or less. (Also it has a shorter range, and you don't get to pick which unit is targeted.)

Comparing Plague Wind with Curse of the Leper, it's a trade-off between high unit numbers and low Toughness. Specifically, Plague Wind is better than Curse of the Leper if the number of models in the unit is 5x(6-T) or more. So 25+ models in a T1 unit, 20+ models in a T2 unit, 15+ models in a T3 unit, and so on. (Happily that's an easy numerical progression to memorise, if you prefer to do so.)

Here's a handy diagram of which power is best (ignoring the limited range and targeting of Curse of the Leper):

Pandemic Staff

The most important complication to factor into our calculations is the Pandemic Staff. This is a Death Guard relic that allows a psyker to add 1 to their Psychic test when attempting to manifest the Smite power.

Using the Pandemic Staff:

Smite has a 27/36 chance of D3 (mean 2) wounds, and a 6/36 chance of D6 (mean 3.5) wounds. That gives an expected number of 2.08(3) mortal wounds.

Plague Wind is then exactly as good as Smite for a unit of 15 models, and better than Smite only if the target unit contains 16 or more models.

Curse of the Leper is then better than Smite only if the target unit has a Toughness of 2 or less.

And again, here's the equivalent illustration:

Note that if you're using a Pandemic Staff, this has some synergy with the Malignant Plaguecaster. This psyker character has the Pestilential Fallout ability that inflicts an additional mortal wound to the closest enemy unit within 7", upon successfully manifesting a psychic power with a Psychic test of 7+. Using the Pandemic Staff with Smite increases the chance of this power triggering from 58.(3)% (21/36) to 72.(2)% (26/36).